In plate-making a lithographic printing plate, a so-called gum solution was coated on the printing plate in the final step thereof.
The purpose of coating such a gum solution is not only for protecting the hydrophilic property of non-image regions of a lithographic printing plate but also for an image correction such as addition, deletion, etc., in image regions, a preservation of lithographic printing plate in the period of from making plate to printing or until reusing the printing plate, protection of a lithographic printing plate from the occurrence of stains and scratches caused by attachment of fingerprints, fats and oils, dusts, etc., in mounting the printing plate on a printing apparatus and during handling the printing plate, and further for restraining the occurrence of oxidation stains.
Hitherto, as a gum solution for a lithographic printing plate, an aqueous solution of gum arabic, a cellulose gum, or a water-soluble high molecular weight material having a carboxyl group in the molecule has been used. However, those gum liquids have had the following problems. That is, usually, in the final finishing step of a printing plate, a gum solution is poured on the printing plate, the gum solution is spread over the entire surface of the printing plate with a sponge, a cotton tampon, etc., and further the surface of the printing plate is rubbed with a wiping cloth until the surface of the plate is dried. In this case, the coating of the water-soluble high molecular weight material, etc., is partially thickened at the image region (ink-receptive region). The thick-coated image portion has a poor inking property at the step of printing and a considerable number of test prints is required until a print having a desired ink density is obtained. Generally, this phenomenon is called blinding (so-called poor inking, that is, a phenomenon that an ink does not attach to the portions of the printing plate to which an ink must be attached).
JP-A-55-19504 (the term "JP-A" as used herein means an "unexamined" published Japanese patent application) discloses an emulsion type surface protective agent for lithographic printing plate containing a plasticizer in an oil phase. However, a satisfactory inking property is not obtained.
On the other hand, recently, there is a tendency to consider that safety is important in the point of environment, and a surface protective agent for lithographic printing plate having a high safety has been desired. Also, it becomes an important theme to decrease the amount of a waste liquid and thus a plate making process wherein after developing, the surface protective agent for lithographic printing plate is coated on the printing plate without washing with water has become a main process. Under such circumstances, a novel surface protective agent for printing plate having a good coating property and a good durability and having imparted thereto a running aptitude has been desired. In particular, a development of a surface protective agent for lithographic printing plate having a high long stability to the contamination of developer components has been demanded.